Yes; standards are typically in fractional inches, or in whole millimeters (e g, 1/4", or 3mm, or whatever.)Get a good set of calipers, and measure your output shaft diameter. Note that 1/4" will be 6.35mm, whereas 6mm will be 6.35mm.Also note that holes are often "press fit" which means it will be very tight -- you may need an arbor press or similar to actually mount the gear.Anything with a set screw in it will not have that problem; there will be slop (slip fit) but I don't know of a series of pinion gears that come with set screws.

The specifications for your servo should specify the output shaft dimensions, btw. Reading that would be safer than measuring yourself.

the picture attached, didnt seem to indicate the shaft dimension, ive also looked for the datasheet but it didnt mentioned anything about the shaft measurement. i guess i just have to measure it myself.

That's not a shaft, that's a spline. And, looking at your ruler, it looks like the outer diameter of the spline is about 5.5 mm.

I think the best thing you can do is to get a round servo horn that is known to fit, and then put (glue? expoxy? screw?) a gear of some sort onto that horn. integy.com has a number of round servo horns for 23T, 24T and 25T splines. Do you know which of those your GWS servos are using? (Google ought to know, at least)

Ahh...a gear output instead of a horn. ok.Start googling and looking at any company that sells servos and Robot parts. I recalled one of them have gears that fit onto servo splined shafts.Also be sure to check with the companies that advertize on this web site (on the right side of this screen).